Aluminum Weld Fatigue Calculation Curve Proposal

FAConle

Updates: Feb 10, June 7-11 2011, Jan.2012
Copyright (C) 2011,2012 F.A. Conle and Univ. of Waterloo
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A copy of the license is available here: "GNU Free Documentation License".
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Introduction:
Recent approaches in the prediction of fatigue life of welded
structures have used the concept of local "hot-spot" stress or
strain near the weld region of interest to quantify the magnitude
of the fatigue loading. Previous guidelines for bridges and other
structures incorporated a series of parallel fatigue life design S-N
curves, where each curve is a function of weld detail severity. The
local approach was implemented to overcome the difficulty of selecting
the weld severity type in complex geometry structures.

In the ground vehicle industry much of the local stress analysis is
done using elastic finite element (or simple stress*Kt) type calculations.
These elastic stresses are then transformed into local stresses and strains
using some form of Plasticity correction tool which requires a definition
of the cyclic stress-strain curve along with the fatigue life curve.
The analysis method proposed here allows this type of correction to be
made for aluminum weld data sets, and thus will conform to the standard
methods presently applied in the ground vehicle industry.

Base Metal Data:

Aluminum AA6061 was selected as the base metal starting point for a
Leever[22] type model of fatigue of aluminum welds. Although there is quite
a bit of information available for base metal AA6061 fatigue curves, not
a great deal of it has been made public. After the creation of the
SAE J1099 database, an event which in my opinion gave a major impetus to
fatigue analysis in the ground vehicle industry, most of the original
contributing corporations "clammed up" and kept further testing to themselves.
In my opinion this lack of openness is generally harmful to all
concerned. My commendations to NRIM (National Res. Inst. for Metals) in
Tokyo, Japan for continuing to "do the right thing" for engineering.

Thus far I have only found two public sets of AA_6061 stress-strain-life
fatigue results:

The data sets are also available in the following location:
http://fde.uwaterloo.ca/Fde/Materials/Alum/AA6xxx/aa6xxx.html
Unfortunately they do not have any results beyond about 50000 reversals to
failure. In order to obtain stress-strain and fatigue curves for longer
lives it was necessary to use the Neuber plot from the paper:

[3] F.A.Conle and J.J.F.Bonnen, "Using the Neuber Plot to Account for the
Effects of Scatter, Corrosion and Welding in Strain-Life Fatigue Test Data,"
2008 ISOPE Conf. Vancouver BC, July 10.

Neuber Stress = SQRT(StressRange * StrainRange* Modulus ) is depicted
versus fatigue life in a combined plot of data from [1],[2], and [3].
By using the elastic behavior of aluminum at longer fatigue lives it was
possible to add the long life information to a merged "Fitted" file
available here:
See: AA 6061- Merged Room Temp data files __ | __ Fitted __ | __ Calculator
The fitted curve points and the data from the three references are
depicted in the Neuber plot:

Click image to enlarge

The fitted curve, or one like it, will form the material "base line"
curve for the Leever method applied to welded aluminum components.

Weld Results to date:

After scanning a typical aluminum weld fatigue curve figure and digitizing
the data one must compensate somehow for the effects of tests run under
different load ratios, in order to plot the data on a combined Neuber type
plot. Thus far I have found that using:

SequivAmpl = SQRT( Smax * Sa )

where SequivAmpl is an equivalent constant R=-1 stress amplitude, Smax
is the maximum stress in the test cycle, and Sa is the stress amplitude of
the test cycle. It is a form of the Smith-Watson-Topper mean stress
correction factor and should work reasonably well under primarily elastic
conditions, which these aluminum weldment tests appear to have.

In some of the graphs below I have not yet deleted the regions of the test
results where one would expect substantial plasticity; typically lives shorter
than 10**4 cycles. I have also left in Sanders' AA3003 data curve but have
not yet obtained the original reference work. Also still missing are
2000 series alloy results. The first three graphs are provided to show the
evolution of the results as more curves are added to the plot.

Click image to enlarge

Click image to enlarge

Click image to enlarge

Finally, all of above data accumulated to date are plotted here:

Click image to enlarge
Figure: Neuber plot of aluminum weld data along with un-notched AA_6061
base line data. Also included is a stress-life plot of the IIW1823 weld curve.

A Comparison to Cast 319 Aluminum:

Given the porosity found in many weldments, it was thought to be of
interest to compare the fatigue behavior of a typical cast aluminum,
in this case a 319 engine material. The baseline data for both simple
constant amplitude strain control tests and periodic overload tests
can be found
here.
A summary figure for cast 319 aluminum is shown in the following graph.

In the above figure the filled brown circles are for 319 constant amplitude
tests and filled square points are 319 periodic overstrain test results.
One can conclude that this particular cast aluminum is very similar to weld
aluminum fatigue data and that the IIW1823 variable amplitude design curve
is similar to the periodically overstrained cast 319 fatigue life curve.

Tentative Design Procedure:

The data from the previous figure was replotted with the inclusion of
the base material "Fitted" Neuber Product reference curve and the same reference curve
with Yaxis terms divided by a factor 3; which amounts to a Kt=3 effect.

(Click image to enlarge)
Figure: Neuber plot of all found aluminum weld data along with un-notched

It consists of the
Fitted base metal calculator with the setting
of the elastic magnification factor to a value of 3 to reflect
a Kt=3. This magnification can be changed at the discretion of the user.

Summary:

It appears that the Leever method can be applied to fatigue life
calculations for aluminum weldments. With base material curve for a merged file of AA_6061
files and a stress concentration factor of Kt=3 most of the
existing sample test results are encompassed. The exception of Sanders'
AA_3003F data set remains a question for further study.

The data provided by Köbler used strain gages close to the fusion weld
toe and appears to be closer to the base material curve. It is not
clear, however, if this closer proximity is due to the use of gages
or because of a higher strength material effect. It would be useful for
FD+E members to perhaps try the strain gage method with Köbler type
specimens on one of the lower strength materials, such as AA_3003 perhaps.

Constant amplitude cast 319 aluminum fatigue life test data falls into the
same band of life results as the welded aluminum tests.

The IIW1823-7 curve appears to also be a good lower bound for much of
the welded results, but is perhaps too low at longer life for the
constant amplitude weld sample tests. A part of this effect
is attributed to test results with mixtures of small and
large cycles, which in other literature has been termed "periodic
overload effect." It appears that there is a correlation between the
cast 319 aluminum periodic overstrain curve and the IIW1823-7 design curve.

Some confirmation tests using variable amplitude histories are also
necessary to determine the validity of this approach to service load
fatigue design.